Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and portland cement, are an essential ingredient in concrete.
Fine and Coarse Aggregates
Characteristics of Aggregates
- Particle shape and surface texture
- Abrasion and skid resistance
- Absorption and surface moisture
- Unit weights and voids
Grading is the distribution of particles of angular materials among various sizes. Grading is usually expressed in terms of cumulative percentage passing each sieve.
Different standards and specifications specify grading limits for both fine and coarse aggregates. There are several reasons for specifying grading limits and maximum aggregate size, they affect relative:
- Relative aggregate proportions
- Cement and water requirement
- Pump ability
- Shrinkage and durability of concrete
- The fineness modulus (FM) for both fine and coarse aggregates is obtained by adding the cumulative percentages by mass retained on each of a specified series of sieves and dividing the sum by 100.
- The FM is an index of the fineness of the aggregate. The higher the FM, the coarser the aggregate. FM of fine aggregate is useful in estimating proportions of fine and coarse aggregate in concrete mixtures.
Particle Shape and Surface Texture
- The shape and surface texture affect the properties of fresh concrete more than the properties of hardened concrete.
- Rough-texture, and angular particles require more water to produce workable concrete than do smooth, rounded and compact particles. For both crushed or noncrushed aggregate, proper gradation gives the same strength for the same cement factor.
- Bond between cement paste and a given aggregate generally increases the particles surfaces change from smooth and rounded to rough and angular. The increase in bond is important for selecting aggregates for concrete where strength at early age is important.
- Aggregate should be free of flat or elongated particles. Because they require an increase in mixing water and thus may affect the strength of concrete particularly in flexure.
Absorption and surface moisture
Absorption and surface moisture of aggregate are measured when selecting aggregate because the internal structure of aggregate is made up of solid material and voids that may or may not contain water. The amount of water in the concrete mixture must be adjusted to include the moisture conditions of the aggregate.
Abrasion and skid resistance
Abrasion and skid resistance of an aggregate are essential when the aggregate is to be used in concrete constantly subject to abrasion as in heavy-duty floors or pavements. Different minerals in the aggregate wear and polish at different rates. Harder aggregate can be selected in highly abrasive conditions to minimize wear.
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